The specific purpose of the proposed research is to gain a better understanding of the role of stimulus equivalence in the extension of stimulus functions to novel stimuli. To the extent that such extension of function is characteristic of symbolic functioning, the broad aims are to gain a greater understanding of how such competencies arise, are maintained, and participate in the organization of human behavior. The research proposed in this application wil determine whether equivalence classes 1) are necessary for the extension of stimulus functions and 2) mediate the maintenance of such extensions. Experiment 1 will train and assess equivalence class formation and the extension of stimulus functions simultaneously, allowing a more precise characterization of the relationship between the two phenomena. The extensio of stimulus functions after equivalence class formation would suggest that equivalence classes mediated the transfer whereas observed transfer of function prior to equivalence class formation would call the mediating role of eqivalence classes into question. The second experiment will determine whether equivalence relations among stimuli mediate the maintenance of extended stimulus functions by changing equivalence relations among stimuli. A change is stimulus functions following a realignment of the equivalence relations would suggest that equivalence classes continue to mediate the extension of stimulus functions. No change in functions may suggest that the two kinds of activity can become independent of one another.